A known technique for joining a circuit board and an electronic component is “solder self-assembly” using “particle agglomeration to junction electrodes by a liquid surface tension”.
Referring to FIGS. 17A to 17E, the solder self-assembly as disclosed in Japanese Patent No. 3955302 and so on will be described below.
In order to join a component 5 to electrodes 2 formed on a circuit board 1 as shown in FIG. 17E, bonding resin 4 containing solder powder 3 is supplied onto the electrodes 2 formed on the circuit board 1 as shown in FIG. 17A. As shown in FIGS. 18A and 18B, the electrodes 2 are formed in an area layout so as to be opposed to electrodes 6 of the component 5 to be joined.
The bonding resin 4 is supplied by typical supply methods of viscous materials. The supply methods include dispensing, screen printing, and transfer. As shown in FIGS. 19A and 19B, the bonding resin 4 is supplied over the electrodes 2 on the circuit board 1.
In FIG. 17B, the component 5 is held by a suction tool 7 above the circuit board 1 placed on a stage 9. The electrodes 6 are positioned in opposition to the electrodes 2, and the component 5 is held with a gap H between the electrodes 2 and the electrodes 6. In this state, at least one of a heater 8 provided in the suction tool 7 and a heater 10 provided in the stage 9 is operated to heat at least one of the component 5 and the circuit board 1 with a predetermined temperature profile, so that the solder powder 3 in the bonding resin 4 melts as shown in FIG. 17C. Further, in the gap H between the electrodes 2 and the electrodes 6 and around the gap H, convection Z is formed by molten solder 12, which is the agglomeration of the molten solder powder 3, and the resin components of the bonding resin 4, so that the molten solder 12 self-assembles between the electrodes 2 and the electrodes 6 due to particle agglomeration to the electrodes by a liquid surface tension.
After that, the circuit board 1 and the component 5 are cooled to the melting point of the solder powder 3 or lower, so that as shown in FIG. 17D, the molten solder 12 self-assembled between the electrodes 2 and the electrodes 6 solidifies and solder metal joints 13 are made between the electrodes 2 and the electrodes 6.
Reference numeral 11 denotes the resin components of the cured bonding resin 4. The resin components act as an underfill for protecting the junctions between the component 5 and the circuit board 1 and obtaining a bonding strength after the solder joints are made.